1. Field of the Invention
The present invention relates to the evaluation of downhole formations from the in situ determination of thermal properties. More particularly, the present invention relates to the in situ determination of thermal properties, such as specific heat, thermal conductivity, and thermal diffusivity from wellbore temperature measurements. Still more particularly, the present invention relates to the in situ determination of thermal properties performed while utilizing a heat source employed in wellbore stabilization, drilling, or perforating.
2. Description of the Related Art
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes the characteristics of the earth formations traversed by the wellbore, and the location of subsurface reservoirs of oil and gas. Samples of the formation and reservoir may be retrieved to the surface for laboratory analysis. However, to enhance drilling and production operations, it is much more valuable to provide real-time access to the data regarding downhole conditions. Thus, it has become commonplace to make in situ measurements of many formation properties via wellbore logging tools, such as logging-while-drilling (LWD) and wireline tools, that may be operated by electromagnetic, acoustic, nuclear or electromechanical means, for example. These logging tools enable in situ determinations of such properties as the porosity, permeability, and lithology of the rock formations; reservoir pressure and temperature in the zones of interest; identification of the fluids present; and many other parameters.
It has been suggested that thermal properties of the formation, such as thermal diffusivity, thermal conductivity, and specific heat (or specific heat capacity), are valuable for determining rock and fluid properties. Thermal diffusivity enables a determination of rock characteristics, such as whether the formation comprises sand, limestone, shale, or granite, for example. Specific heat enables a determination of fluid properties, such as whether the formation is saturated with hydrocarbons or water. In particular, the specific heat of water is approximately twice the specific heat of a hydrocarbon, regardless of whether the hydrocarbon is a liquid or gas. Thermal conductivity enables further differentiation between liquid and gas hydrocarbon-saturated formations. In particular, the thermal conductivity of a water-saturated formation is approximately twice the thermal conductivity of an oil-saturated formation, and the thermal conductivity of an oil-saturated formation is approximately twice the thermal conductivity of a gas-saturated formation.
Thermal conductivity (K), the physical property of a material that determines how easily heat can pass through it, is defined by equation (1), which relates several thermal properties as follows:K≡ρ·C·κ  (1)where ρ is the material density, C is the specific heat capacity, and κ is the thermal diffusivity. Specific heat capacity (C) is defined as the quantity of heat required to raise the temperature of one unit of mass of material by one temperature degree. Thus, the units of specific heat capacity (C) may be calorie/gram° C., for example. Thermal diffusivity (κ) is defined as the rate at which heat is conducted during unsteady state heat transfer.
To make thermal property determinations, the most common method is to take a sample of the formation downhole, retrieve it to the surface, and then evaluate the sample in a laboratory. The laboratory method does not enable real-time data acquisition, and can only be accurate with respect to the particular sample acquired. Since formation properties typically change with depth, it is likely that the laboratory analysis will not provide complete data for all zones of the formation. Other methods include inferring thermal properties from ambient borehole temperatures measured by conventional wellbore logging tools. This method has some limitations, including the inability to make accurate measurements through casing to determine formation properties behind the casing and the cement that surrounds it.
Yet another known method for determining thermal properties is to provide downhole a constant output heat source and to measure the temperature relaxation over time, which enables accurate measurements in both cased and uncased boreholes. U.S. Pat. No. 3,807,227 ('227 Patent) and U.S. Pat. No. 3,892,128 ('128 Patent) to Smith, Jr. disclose such thermal well logging methods for determining specific heat and thermal conductivity, respectively. A single heat source and preferably three spacially separated thermal detectors are disposed on a well logging tool that is moved vertically through a borehole while the thermal responses are recorded. In particular, one thermal detector measures ambient temperature of the borehole at a particular depth before the heat source passes that depth. The other two thermal detectors measure the temperature of the borehole at the same depth following the heat source, each at a different time. In one embodiment, the heat source is disclosed to be a heat drilling tip that melts the earth formations to produce a borehole. Examples of modern heat drilling tips are disclosed in U.S. Pat. No. 5,735,355 to Bussod et al. comprising a rock melting tool with an annealing afterbody that cools the molten rock, and U.S. Pat. No. 5,771,884 to Potter et al. comprising a spallation head with rotating, circumferentially spaced jets that dispense flame jets, very hot water, and/or air to spall the formation rock, or fuse the formation rock if spallation is not feasible.
The '227 Patent discloses that in a borehole environment, the change in temperature (ΔT) is related to the radiated energy (Q) from the heat source, the mass of the heated earth formation (M) and its composite specific heat (C) as given by the relationship of equation (2):Q=C×M×ΔT  (2)
Similarly, the '128 Patent discloses that thermal conductivity (K) is proportional to the time rate of heat transfer in the formation. The one dimensional relationship governing the energy transfer (ΔQ) during a short period of time (Δt) in a formation having a temperature differential (ΔT) over a length (ΔX) is given by Equation (3):
                                          Δ            ⁢                                                  ⁢            O                                Δ            ⁢                                                  ⁢            t                          =                  K          ×          α          ×                                    Δ              ⁢                                                          ⁢              T                                      Δ              ⁢                                                          ⁢              X                                                          (        3        )            where α is a constant dependent on the geometry of the borehole, the formation, and the well logging tool; and K is thermal conductivity. Thus, according to the '227 Patent and the '128 Patent, the specific heat (C) and the thermal conductivity (K) of the formation can be inferred using equation (2) and equation (3), respectively, based on temperature measurements. Using these inferred values, qualitative evaluations of likely locations of water and hydrocarbon deposits can be made.
U.S. Pat. No. 3,864,969 ('969 Patent) to Smith, Jr. discloses two methods for determining thermal conductivity (K) of the formation by heating one spot within the formation. In the first method, the formation is heated for a predetermined length of time to elevate the temperature. Then the heat source is removed and the rate of temperature decay is measured over time until the formation returns to ambient temperature. In the second method, the formation is heated by a constant output heat source and the formation's rate of temperature increase is measured to derive an indication of thermal conductivity (K).
Similarly, U.S. Pat. No. 4,343,181 (the '181 Patent) to Poppendiek discloses a method for in situ determinations of the thermal conductivity and thermal capacity per unit volume of the earth. The '181 Patent teaches a probe containing a heater and two temperature sensors spacially displaced from one another. The probe is positioned in the borehole at the level of interest and maintained at that position for a period sufficient for the probe to be in thermal equilibrium with its surroundings. The probe is displaced from the borehole wall by a thin fluid annulus, and it is not in contact with the borehole wall. The thermal gradient between the two temperature sensors is recorded without heat being applied. Then, the heater is turned on to apply heat at a constant rate, and the thermal gradient between the temperature sensors is recorded. The thermal conductivity and thermal capacity per unit volume of the surrounding earth is determined by relating the actual temperature curve to a calculated theoretical curve by best-fit mathematical methods. At short times, the thermal capacity is said to dominate the temperature response curve, and at long times, the thermal conductivity is said to dominate.
Each of these prior in situ methods proposes utilizing a downhole heat source that is provided for the sole purpose of taking thermal measurements. Although this approach is technically sufficient, and valuable formation characteristics can be determined using this methodology, this approach has largely been ignored in practice. One possible explanation is that operators are not willing to incur additional capital and operating costs for a heat source that is provided solely for thermal property measurements. Thus, most commercial downhole systems do not include heat sources that enable in situ measurements of thermal properties. Accordingly, at the present time, thermal property measurements are almost exclusively restricted to analysis of samples in laboratories.
Further, although the '128 Patent and the '227 Patent mention the concept of a heat drilling tip that may also be used as a heat source for enabling in situ determinations of thermal properties, such heat drilling tips have proven to be too slow for commercial success. In particular, the heat drilling tip is designed to spall or actually melt the rock of the formation as the method of forming a borehole. However, because rock is very slow to spall or melt utilizing such techniques, the heat drilling tip progresses at only 3-6 feet per hour. Therefore, the heat tip has not achieved commercial recognition or success as a viable drilling alternative.
The present invention addresses the deficiencies of the prior art by providing a convenient in situ method of measuring formation thermal properties, such as specific heat, thermal conductivity, and thermal diffusivity. The method is suitable at multiple depths using a commercially viable heat source provided downhole for wellbore stabilization, well drilling or well perforating.